The present invention relates generally to binding agents useful in the selective depletion of T cells in vivo. More specifically, the invention relates to ICOS-binding agents which once bound to ICOS expressed on the surface of cells, in particular ICOS-bearing T cells, result in the in vivo depletion of cells to which they are bound. Methods of treating T cell related diseases using said ICOS-binding agents, pharmaceutical compositions comprising said ICOS-binding agents, a method of identifying an ICOS-binding agent, and monoclonal anti-ICOS antibodies capable of in vivo eliminating cells which express ICOS on their surface are also provided.
The concept of depleting T cells in autoimmune disease and transplantation is not new and has successfully been practiced in the clinic for decades (Hargreaves R E G, Trends in Molecular Medicine 2004; 10:130-135). However, the antigens which were targeted on T cells (CD3, CD4, CD52) are all broadly expressed on resting and activated T cells or even on other cell types. Therefore, the existing therapeutic regimens are broadly immunosuppressive. The depletion of activated T cells expressing ICOS in vivo, as described in the present invention, offers much higher specificity of the therapeutic effect, since ICOS is only significantly expressed on activated effector T cells (Löhning M, Journal of Experimental Medicine 2003; 197:181-193; Bonhagen K, European Journal of Immunology 2003; 33:392-401).
In the European Patent Application EP1 158 004 A2 human antibodies are described, which can kill ICOS-overexpressing transfectants in vitro based on the mechanism of Antibody Dependent Cell-mediated Cytotoxicity (ADCC) with human peripheral blood lymphocytes. The described experiment demonstrates that the used antibodies can mediate a certain type of in vitro cytotoxicity but does not provide any information whether ICOS-expressing T cells can at all be depleted in vivo. There are a number of reasons, why this experiment cannot provide this information: 1. ICOS overexpressing transfected lines were used as targets and not normal T cells with physiological ICOS expression. 2. ADCC as used in the described experiment has often been demonstrated as a cell-depleting mechanism in vitro, but has been largely excluded as a mechanism of for cell depletion in vivo (Alters S E, Journal of Immunology 1990; 144:4587-4592; Uchida J, Journal of Experimental Medicine 2004; 199:1659-1669). ADCC will work in vitro with any antibody capable of binding to a surface molecule having the right isotype to be recognized by the cytotoxic cells in vitro. 3. There are a variety of additional reasons (see below), why in vitro depletion tests in general have no predictive value for in vivo depletion (Uchida J, Journal of Experimental Medicine 2004; 199:1659-1669, Isaacs J D, Rheumatology 2001; 40:724-738). This is the reason why it has been difficult to generate in vivo depleting antibodies directed against other T cell surface activation antigens like CTLA-4 or OX40, whereas in vitro (e.g. ADCC-based) T cell-depletion against these targets will work fine (Hargreaves R E G, Trends in Molecular Medicine 2004; 10:130-135).
A greater number of factors are important in determining whether a given cell surface molecule can be used as a target for cell depletion in vivo: level of target molecule cell surface expression, target antigen valency, distribution of the targeted antigen on the cell surface, its proximity to the cell membrane, occurrence or not of internalization of the target molecule after antibody binding, course and duration of physiological expression, accessibility of the target antigen in various tissues (lymph nodes, spleen, blood), and other factors (Alters S E, Journal of Immunology 1990; 144:4587-4592; Uchida J, Journal of Experimental Medicine 2004; 199:1659-1669; Isaacs J D, Rheumatology 2001; 40:724-738). These factors and thus the general usefulness of a given molecule as a target for cell depletion can only be tested using an appropriate model in vivo.
Moreover, ICOS as a co-stimulatory molecule poses the danger that antibodies or other agents directed to ICOS lead instead of depletion to an untoward activation and expansion of ICOS+ T cells in vivo with potentially severe disease-aggravating effects. Up to date there are no data available whether T cells can be depleted in vivo using a co-stimulatory molecule as a target. In vitro tests for ICOS co-stimulatory effects do not have a predictive value for the in vivo situation, since many critical parameters cannot be correctly reproduced in vitro (e.g. ICOS surface density and surface distribution, binding characteristics of the ICOS-binding agents to various tissues, etc.). Therefore, the question whether ICOS can be used as a target for a therapeutic depletion of activated T cells in vivo without adverse cell activation and proliferation can only be answered in an appropriate disease model in vivo. Only such an in vivo test has a predictive capacity for the usefulness of ICOS as therapeutic target. At the same time, only in vivo tests have a predictive value for the usefulness of monoclonal antibodies directed to ICOS as depleting agents.